First, a configuration of a conventional microorganism number-measuring apparatus is described.
The conventional microorganism number-measuring apparatus includes: a measurement container, a rotary driver, a bacteria-collection signal generator, a measurement signal generator, an output amplifier, an I/V (current/voltage conversion) amplifier, an impedance measuring unit, a microorganism number-computing unit, and a solution conductivity-computing unit.
The measurement container is such that a measurement electrode is disposed in a measurement liquid inside the container, in an immersed state in the liquid. The rotary driver rotationally drives the measurement container. The bacteria-collection signal generator feeds an alternate-current bacteria-collection signal to the measurement electrode. The measurement signal generator feeds a measurement signal to the measurement electrode. The output amplifier is coupled with an output of the measurement signal generator and with an output of the bacteria-collection signal generator.
Moreover, the I/V amplifier is coupled with an output of the output amplifier via the measurement electrode. The impedance measuring unit is coupled with the I/V amplifier to measure impedance of the measurement liquid. The microorganism number-computing unit is coupled with the impedance measuring unit. The solution conductivity-computing unit is coupled with the impedance measuring unit.
In the conventional microorganism number-measuring apparatus, the microorganism number-computing unit computes the number of microorganisms based on the impedance measured by the impedance measuring unit (see Patent Literature 1, for example).
When using such the conventional microorganism number-measuring apparatus to measure the number of microorganisms (bacteria) present in an oral cavity, variations in measurement accuracy appear due to variations in electric drifts which occur in a system of measurement.
Of the electric drifts of the system of measurement, it is a particularly problematic drift that occurs in the I/V amplifier for converting an electric current flowing through the measurement electrode into a voltage. In the I/V amplifier, heat associated with use is generated to cause a large drift in the system of measurement. Variations in the drift appear at a certain rate corresponding to the magnitude of the drift. The variations in the drift will possibly provide adverse effects on measurement accuracy.